<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>

<body>
    <script>
        var a = 10
        function test() {
            a = 100
            console.log(a)  // 100
            console.log(this.a)  // 10
            var a;
            console.log(a)  // 100
        }
        test()	
    </script>
</body>

</html>